Insulating refractory material

ABSTRACT

The invention concerns a crack-free insulating refractory material made from a composition comprising 20-80% by weight of a ceramic matrix, 5-40% by weight of insulating microspheres, 0.5-15% by weight of one more binders, 5-20% by weight of a metal or a metal alloy able to melt during the preheating or the first minute of use and 0-25% by weight of water. The ceramic matrix is preferably comprised of vitreous and non-vitreous grains.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an insulating refractory material and a methodof making the material, and more specifically to an insulating coatingfor use in the casting of molten metals.

2. Description of the Related Art

Insulating refractory materials are used in industry, notably inmetallurgy, for reducing heat losses and for saving energy. Thesematerials can also be used for coating an article. They can also be usedfor producing an insulating component in the material itself. They canalso be used to produce elements such as panels or bricks that will beused severally to form an insulating unit.

In the continuous casting of steel, refractory components are used fortransferring molten steel between various containers, notably betweenthe ladle and the distributor, and the distributor and the continuouscasting mold. Thermally insulating these components can, for example,improve the preheating efficiency (when the components are preheated),reduce solidification of the steel on the inside walls of the tapholeand, in the case of pouring components used for a distributor, decreasebridging between the casting component and the mold walls. In oneexample, a sleeve of insulating refractory material fitted on a stopperrod can improve the preheating efficiency by retaining the heat of theburner.

Typically, sheets or mats of paper impregnated with ceramic fiberscomprise the insulating refractory material. Although providing goodthermal insulation, ceramic papers have several shortcomings. Placementof the ceramic paper requires operations of cutting, placement andgluing that are long and tedious. Furthermore, the handling of sheets orplates of ceramic paper permits the escape of carcinogenic ceramicfibers that can be inhaled by the operators.

An insulating refractory coating for steel casting components is alsoknown (EP 0 296 981). This coating is obtained from a composition of anaqueous suspension containing 30-85% by weight of a finely dividedconstituent such as fused silica, alumina or zirconia powders andalumina beads, and a ceramic charge of fibers of alumina, silica,zirconia, titanium dioxide or chrome-alumina, or even alumina orzirconia beads. This composition is also comprised of up to 7% by weightof a binder such as sodium hexametaphosphate or sodium silicate, and upto 40% by weight of a glass-generating frit.

Such a coating avoids some of the disadvantages of plates of ceramicpaper impregnated with ceramic fibers. In particular, it is more rapidto install because it does not require a large number of operations. Inaddition, it can facilitate avoiding the presence of ceramic fibers thatare dangerous for the health of the operators. However, it presentscertain disadvantages. Its thermal insulation characteristics are notvery good due to its low porosity (ca. 20%). This porosity is alsoessentially open, which assures less good thermal insulationcharacteristics than a closed porosity. Furthermore, it is difficult todeposit a substantial thickness of coating on a casting. To increase thethickness of the coating, it is necessary to preheat the componentsbefore coating them, which requires a supplementary stage and implies asupplementary cost. Furthermore, after having deposited a first layer,it is not possible to deposit a second layer because the outer surfaceof the coating is smooth and impermeable, which does not permit a goodadhesion of the second layer.

SUMMARY OF THE INVENTION

The object of the present invention is an insulating refractory materialthat remedies these shortcomings. Another object of the presentinvention is to provide an insulating refractory material which does notpresent cracks or micro-cracks after the preheating or when used.

The insulating refractory material is particularly suitable for thepreparation of insulative coatings for relatively wide refractoryarticles such as for example so-called fish-tail pouring nozzles usedfor the continuous casting of thin slabs. It has indeed been observedthat due to severe surface constraints appearing when such an article isheated, the coating may be subject to peeling.

Such a peeling or a deterioration of the insulating coating must beavoided at all costs. Firstly, an underlayer of protective material(such as a glaze), if present, could be deteriorated by the peeling ofthe surface layer. In turn, the degradation of the protective layergives rise to the oxidation of the refractory article constituting thearticle. Secondly, the partial peeling of the insulative coating exposesportion of the coated article to extremely high temperatures while theremaining portions are still protected and insulated. The thus generatedtemperature gradient is responsible of an important thermal shock thatmay damage the article.

DETAILED DESCRIPTION OF THE INVENTION

This insulating refractory material is comprised of 20-80% by weight ofa ceramic matrix, 5-40% by weight of insulating microspheres, 0.5-15% byweight of one or more binders, 5-20% by weight of a metal or metal alloyable to melt during the preheating or the first minute of use and up to25% by weight of water. The matrix can be a matrix of vitreous grains,notably silica, preferably atomized silica; and may also comprisenon-vitreous grains such as alumina or magnesia. Preferably, the matrixdoes not comprise more than 30% by weight of the matrix of non-vitreousgrains. Preferably, the matrix comprises between 5 and 20% by weight ofnon-vitreous grains such as alumina or magnesia.

The metal or metal alloy that might be used according to the inventionmust be able to melt during the preheating of the coated article (whenthe article is preheated) or the first minutes of use of the coatedarticle so that the liquid or semi-liquid metal or metal alloy mayimpregnate the porosity and (micro)voids within the coating. Commonly,the metal or metal alloy will also be oxidized and form metal oxide insitu. According to a particular embodiment of the invention, the metalis therefore selected so as to form upon oxidization a refractory metaloxide. Suitable metals and metal alloys include aluminum, aluminumalloys (such as AA1100, AA5052, etc.), copper, brass, manganese bronze,zinc, and the like. Among these, aluminum is preferred.

The metal or metal alloys may be incorporated in the form of grains orflakes, preferably as grains having an average grain size up to 0.2 mm.The material may contain from 5 to 20% by weight, preferably from 8 to15% by weight, and more preferably about 10% by weight of metal or metalalloy. It has been observed that the addition of metal or metal alloyincreases the thermal conductivity of the final coating and maynegatively impact on its insulating properties. Surprisingly, it hasbeen found that these opposite requirements of having a crack-freecoating showing good insulating properties could be balanced byselecting an appropriate amount of metal or metal alloy.

To avoid corrosion or aging of the metal or metal alloy in the coatingbefore its use and to increase consequently the shelf life of the coatedarticle, it may be necessary to include an anti-oxidant and ananti-aging agent. The refractory material may comprise up to 0.2% byweight of an anti-corrosion agent. Particularly suitable agents aresodium tripolyphosphate or tetrasodium dipolyphosphate, but otherconventional anti-oxidant agents may also be used. Preferably, aconventional anti-aging agent is also used in a conventional amount. Thematerial can also have up to 4% by weight of a deflocculant and up to20% by weight of colloidal silica. Preferably, it has from to 0.5 to 4%by weight of a deflocculant and from 0.5 to 20% by weight of colloidalsilica.

The insulating microspheres may be hollow spheres of a refractorymaterial, which is typically based on silica and/or alumina. In apreferred embodiment, the microspheres comprise 55-65% by weight ofsilica and 27-33% by weight of alumina.

The binder of the invention impacts the rheology of the insulatingmaterial, particularly as an aqueous suspension or slip. The rheology ofthe insulating material affects its method of application. For example,a particular rheology is required for applying a slip of the insulatingmaterial by dipping or immersing the refractory piece into the slip. Theformation of a regular and homogeneous coating depends indeed upon theviscosity of the slip. A suitable viscosity for dipping applications isgenerally higher than 8 Pa.s and preferably higher than 10 Pa.s. Bindersthat might be used according to the invention are clays of the kaolinitetype and organic binders such as the polysaccharides (e.g., dextrine).

The invention also concerns a component, notably for casting steel,having a body of refractory material coated with the insulating materialof the invention. The component can also be a composite piece producedpartially of the material of the invention. This component can beproduced, e.g., cast, in a single operation or formed of severalassembled pieces.

The invention also concerns a process for preparing a composition foreffecting an insulating coating or making an insulating piece. Accordingto this process:

one or more binders are dissolved in a quantity of water;

a deflocculant is added;

grains of the ceramic matrix (including grains of atomized vitreoussilica) are added while agitating the solution to hydrate them and forma slip;

an anti-corrosion agent is added;

microspheres of an insulating material and a metal or metal alloy areadded while continuing to agitate the slip to keep it homogeneous.

In a preferred variant of the process, colloidal silica is added afterthe deflocculant.

The composition used in making the material can be a slip containingfrom 20 to 70% by weight of atomized vitreous silica grains, from 5 to40% by weight of insulating 20 microspheres, from 0.5 to 20% by weightof one or more binders, from 3.0 to 15.0% by weight of metal or metalalloy and from 5 to 25% by weight of water. The composition may furthercomprise up to 4% by weight of a deflocculant, up to 0.15% by weight ofan anti-corrosion agent and up to 10% by weight of colloidal silica.Preferably, it further comprises from 0.5 to 4% by weight of adeflocculant and from 0.5 to 10% by weight of colloidal silica. Such acomposition may have a viscosity of between 9 to 12 Pa.s.

The invention also concerns a refractory component having the describedinsulating coating. The invention also concerns a process for coatingthe refractory component used in the casting of molten metal,particularly steel with the composition of the invention. The process ofcoating the refractory component includes dipping the component in acomposition as described above for a time less than one minute andallowing it to dry in the open air for 2-4 hours. Repeating the processmay produce a plurality of insulating layers.

The material of the invention presents numerous advantages over ceramicpapers, including ease of application and reduction of hazardous fibers.Additionally, it assures a better thermal insulation because theinsulating microspheres provide a greater porosity and a closed porestructure. Furthermore, the quality of the thermal insulation isimproved because it is possible to deposit a greater coating thicknesson the piece without impairing the insulation characteristics. Thisthickness can range up to 4 mm in a single layer and up to 7 mm in twolayers. Furthermore, the coated articles will present very few cracks ormicro-cracks, even after preheating or after the first minutes of use.Finally, it is readily possible to produce insulating pieces totally orpartially of this material.

Other characteristics and advantages of the invention will becomeevident from the following detailed description and the implementationexamples.

EXAMPLE I

A slip was prepared having the composition described below. Theingredients were added under continuous agitation by a COUVROT-LAINEbrand planetary-type mixer.

Water 12.1% Dextrine 2.9% Colloidal silica 7.8% Dolapix CE 64 1.7%Fillite SG 500 8.6% Clay (HYMOD RF CLAY) 4.1% Atomized silica 42.9%Alumina 10.7% Aluminum (metal) 9.1% Sodium tripolyphosphate 0.1%

Dextrine, an organic binder, was dissolved in the water. Clay of thekaolinite type was added as a second binder and suspension agent.Continuous agitation ensured homogeneity and complete hydration of theclay, thereby avoiding agglomeration. DOLAPIX CE 64, sold by the Germancompany ZSCHIMMER & SCHWARZ AG, was added as a deflocculant. DOLAPIX isa dispersant/ deflocculant for raw materials and ceramic masses based oncarboxylic acid without alkali, particularly destined for thedeflocculation of ceramic oxides, steatites, etc. The aqueous colloidalsilica comprising 30% by weight silica was then added. Grains ofatomized silica were next introduced into the slip. The grains wereobtained from the aqueous colloidal silica. Water was eliminated fromthe colloid by a hot stream of air to form the grains, which are more orless complete microspheres of silica having a size from 50 μm to 1.5 mmin diameter. Alumina powder (having a maximum grain-size lower than 45μm) was then incorporated into the slip. Insulating microspheres ofFillite SG-500, which comprise alumina-silicate with a particle sizebetween 5 and 500 μm, were added. The density of alumina-silicatecomprising the microspheres was between 2.7 and 2.8 g/cm³, but theapparent density of the microspheres was only 0.6-0.8 g/cm³. Theanti-corrosion agent was then added as an aqueous solution and finally,aluminum powder (having a maximum grain-size lower than 200 μm) wasincorporated.

A nozzle having an elongated body comprising an alumina-graphitecomposition was immersed in the slip at a rate of several meters perminute. The nozzle was kept immersed for less than 1 minute, for example10 seconds. The nozzle was withdrawn slowly from the slip at a rate ofless than 3 meters per minute, drained above the slip for less than 2minute, and allowed to dry in air for 2-4 hours. Optionally, a secondcoat or subsequent coats could have been applied in the same mannerafter a dry time of only 45 minutes. The coated nozzle was then driedthoroughly in a kiln at a constant temperature of 100° C. for one hour.

The chemical composition of the coating obtained from this composition,was as follows:

INGREDIENTS % (BY WEIGHT) SiO₂ 70.0 Al₂O₃ 17.7 Al metal  9.0 MgO traceBurning loss  3.1

EXAMPLE II

A preheating sleeve was produced for a tundish stopper rod. The sleevehad a cylindrical part that fits on the outside diameter of the stopperrod and a flared internal part that completely covers the taphole of thedistributor. This sleeve was made of the insulating refractory materialof the invention by pouring the slip of Example I into a plaster mold.

EXAMPLE III

A pouring spout cover plate was made for the glass industry. The platewas a parallelepiped having a thickness of 50 mm. This plate was made ofthe insulating refractory material of the invention by pouring the slipof Example I into a plaster mold.

What is claimed:
 1. An insulating refractory material made from acomposition comprising: a) 20-80 wt. % ceramic matrix; b) 5-40 wt. %insulating microspheres; c) 0.5-15 wt. % at least one binder; d) 5-20wt. % metal adapted to impregnate voids in the material upon heating;and e) up to 25 wt. % of water.
 2. The material of claim 1, wherein themetal is selected from the group consisting of aluminum, copper,manganese, zinc, and alloys thereof.
 3. The material of claim 1, whereinthe composition comprises from 8-15 wt. % metal.
 4. The material ofclaim 1, wherein the composition comprises an anti-corrosion agent. 5.The material of claim 1, wherein the ceramic matrix comprises vitreousgrains.
 6. The material of claim 5, wherein the vitreous grains comprisesilica.
 7. The material of claim 6, wherein silica comprises atomizedsilica.
 8. The material of claim 1, wherein the ceramic matrix comprisesnon-vitreous grains.
 9. The material of claim 8, wherein thenon-vitreous grains are selected from the group consisting of alumina,magnesia and mixtures thereof.
 10. The material of claim 1, wherein theinsulating microspheres comprise hollow spheres comprising silica andalumina.
 11. The material of claim 10, wherein the hollow microspherescomprise 55-65 wt. % silica and 27-33 wt. % alumina.
 12. The material ofclaim 10, wherein the binder comprises an organic binder.
 13. An articlefor casting molten metal comprising a refractory body having an outersurface at least partially comprising by a material made from acomposition comprising: a) 20-80 wt. % ceramic matrix; b) 5-40 wt. %insulating microspheres; c) 0.5-15 wt. % at least one binder; d) 5-20wt. % metal adapted to impregnate voids in the material upon heating;and e) up to 25% by weight of water.
 14. The article of claim 13,wherein a majority of the article comprises the composition.
 15. Anaqueous composition for the manufacture of an insulating refractorymaterial comprising: a) 20-80 wt. % ceramic component; b) 5-40 wt. %insulating microspheres; c) 0.5-20 wt. % of at least one binder; d) 5-20wt. % metal; and e) 5-25 wt. % water.